Conventional imaging systems or microscopes are based on two forms of radiation, electromagnetic waves as in the optical instrument and the electron waves as in the electron microscope.
Optical instruments have been refined over a period of many years to provide more accurate images of even quite small objects such as biological cells. Regardless of such refinements, inherent limitations exist since the optical system basically senses the dielectric properties of the object being imaged. In the first place, resolution, of course, is limited by the wavelength of the visible light (0.5 microns for green light). Some samples are not transparent and therefore inaccessible for viewing. Furthermore, limitations on the contrast sensitivity have appeared and have been but partially overcome by the tedious technique of staining biological specimens to improve such contrast.
Electron microscopes have much greater resolution capabilities but other difficulties arise. The specimen must be viewed in vacuum, a technical problem, and living cells can not be viewed because of the electron bombardment.
The relatively recent development of acoustic wave generation at frequencies approximating 1,000 MHz provides an acoustic wavelength in the neighborhood of one micron and accordingly has suggested itself as a potentially excellent mechanism for the generation of high resolution images and, by way of example, the method described by B. A. Auld et al, "A 1.1 GHz Scanned Acoustic Microscope, " Acoustical Holography 4, Plenum Press (1972) p. 96 has proved not only a high resolution imaging mechanism but also one which provides good contrast sensitivity. However, there has been developed no medium equivalent to a photographic film for recording the image.